Apparatus and method for increasing the selectivity of FET-based gas sensors

ABSTRACT

A FET gas sensor having a relatively low operating temperature, for example, room temperature, is free from cross sensitivities from interfering gases by a preceding in-line filter. The sensor&#39;s service life is substantially stabilizable by using fabric-like activated charcoal filters which can be regenerated by a moderate temperature increase, and by limiting the diffusion of the analyte gas, which is made possible by the relatively small amount of gas detectable on the sensitive layer of the sensor. This substantially increases the service life of the filters. The gas sensor eliminates cross sensitivities to thereby increase the detection reliability thereof. Also, the gas sensor has relative long term stability and is economical to build. The gas sensor can read relatively weak signals generated by gas-sensitive layers, for example, without other stronger gas signals interfering with the weak signals.

PRIORITY INFORMATION

This patent application claims priority from International patentapplication PCT/EP2005/004281 filed Apr. 22, 2005 and German patentapplication 10 2004 019 640.0 filed Apr. 22, 2004, which are herebyincorporated by reference.

BACKGROUND INFORMATION

This invention relates in general to gas sensors and in particular to adevice for increasing the selectivity of gas sensors that employ fieldeffect transistors (“FETs”).

FIGS. 3 and 4 herein and German Patents DE 4239319, DE 19956744, and DE19956806 disclose FET gas sensors of hybrid construction, which read thework function of a gas-sensitive material using a field effecttransistor. These sensors have a number of different applications. Onthe one hand, they can be operated at ambient temperature, or atslightly elevated temperatures, and they therefore permit low-poweroperation with batteries or direct connection to data bus lines withoutthe use of auxiliary power. On the other hand, a large number ofdifferent materials can be used as detection materials with this sensorstructure, so that a previously unachieved bandwidth of gases can bedetected with these sensors. Economical manufacture is possible withreadily automated techniques because of their simple structure. Sincethe control electronics can be integrated into the Si chip with littleadded cost, the costs of gas sensor systems having the controlelectronics are lower than for other sensor technologies.

These gas sensors are also subject to the problems of crosssensitivities; in other words, other gases that exist in the applicationcan cause interference with the sensor signal. That is, the sensorreacts to interfering gases that may distort a concentration of themeasured gas (i.e., direct cross sensitivity). Similarly, sensitivity tothe target gas can be modified by the presence of an interfering gas(i.e., indirect cross sensitivity). Both effects lead to distortion ofthe desired sensed values and can impair or even prevent usability in anapplication, depending on the requirement profile.

A first approach to eliminating existing drawbacks is an intelligentsignal processor on the system level. In this case, the attempt is madeto eliminate the consequences of incorrect measurements by aplausibility consideration for the sensor signal, for example. Thisapproach is not possible with indirect cross sensitivity.

A second approach uses an additional sensor element that is sensitive tothe target gas and corrects the indicated value of the primary sensorelement with this auxiliary information from the additional sensorelement during the test. This is an approach that was actually pursuedwith array-capable sensors (multiple system) like GasFETs; see forexample German Patent DE 19956806. This variant is always associatedwith distinctly greater effort and higher cost. The consequences ofindirect cross sensitivity, however, cannot be eliminated with thisapproach, or only with great effort, for example with large sensorarrays or extensive calibration models.

A third approach comprises further developing and optimizing the sensormaterial so that selective detection of the target gas is achieved. Thiscan be achieved for some specific applications, for example see GermanPatents DE 19926747, DE 19849932, and DE 19708770. However, it cannot beassumed from this that this is possible for most detection tasks.

A fourth approach of the prior art is related directly to the FET gassensors, for example see German Patent DE 19849932. It utilizes thegeometry of this structure to produce very high electric field strengthson the surface of the sensor layer by applying manageable voltages, forexample 10V, to the suspended gate electrode because of the small airgap. These affect the adsorption properties of the detected molecules.The influence of an interfering gas can be eliminated by comparing themeasured signals with various electric field strengths on the surface ofthe sensor layer. This approach is not universally applicable, such thatits utility is limited to a few special cases.

A fifth prior art approach constitutes a self-explanatory procedure foreliminating cross sensitivities. The use of filters that are mountedbetween the gas mixture to be detected and the gas sensor is proposedfor this purpose. They are permeable to the target gas but do not allowgases that cause cross sensitivities to reach the sensor. Exemplaryembodiments in this case are catalytic filters as disclosed in GermanPatent DE 19926747, with the concentrations of interfering gas beingactively removed by a chemical reaction. Gas sensors based on heated,semiconducting metal oxides are often combined with an activatedcharcoal filter. This removes gases that cause cross sensitivities byadsorbing them on the large internal surface area of the filtermaterial, but with the target gas being allowed to pass through thefilter and be detected by the sensor. A widely used example of suchsensors is gas sensors for detecting toxic gases, CO for example, orexplosive gases, for example escaping natural gas/CH₄ in domesticatmospheres. Alcohol vapors occurring in the household often interferewith their measurement signals as disclosed in German Patent DE19926747. Filters are also used frequently for electrochemical gassensors. Activated charcoal is a very good absorber for alcohol vapors,for example, in long-term operation in these applications. However, thefilter can become saturated, so that the filter loses its activity andthe interfering gas ultimately does reach the sensor and is detected.

There is a need for a FET-based gas sensor that prevents distortion ofthe measured signal by cross sensitivities.

SUMMARY OF THE INVENTION

The invention is founded on the recognition that FET-based gas sensorsthat have a very low operating temperature in contrast with the priorart, for example room temperature, and that are freed of crosssensitivities from interfering gases by preceding in-line filters, aresubstantially stabilizable with regard to the service life of a sensor.This is accomplished by using fabric-like activated charcoal filtersthat can be regenerated by a moderate temperature increase, and bylimiting the diffusion of the analyte gas, which is made possible by theextremely small amount of gas detectable on the sensitive layer of anFET gas sensor. This is able to substantially increase the service lifeof filters.

Advantageously, the sensor effectively eliminates cross sensitivities toincrease the detection reliability of gas sensors. In addition, thesensor has long term stability and is relatively economical to build.The sensors make it possible to read weak signals generated bygas-sensitive layers, for example, without the ability of any otherstronger gas signals to interfere with the weak signals. Such sensorshave operating temperatures between room temperature and about 100° C.

These and other objects, features and advantages of the presentinvention will become more apparent in light of the following detaileddescription of preferred embodiments thereof, as illustrated in theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional illustration of a GasFET with a flexibleactivated charcoal filter mounted according to the invention with noseparation;

FIG. 2 is a schematic illustration of the structure for slowing filtersaturation by limiting the entry of gas;

FIG. 3 is a cross-sectional schematic illustration of a prior art GasFEThaving a suspended gate;

FIG. 4 is a cross-sectional schematic illustration of a prior art GasFEThaving a capacitively coupled FET; and

FIG. 5 is a cross-sectional illustration of a gas sensor in which thegas filter is mounted in a depression in the ceramic.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a cross sectional illustration of a sensor system 10 of aGasFET 12 that includes a Si base with source, drain, and channelregions with a suspended gate electrode. The GasFET includes a flexibleactivated charcoal filter 14 mounted with no separation according to theinvention, which encloses the entire sensor system.

FIG. 2 schematically illustrates a structure for slowing filtersaturation by limiting the entry of gas, so that the filter has asubstantially longer service life.

FIG. 3 is a cross-sectional schematic illustration of a prior art GasFETthat includes a suspended gate FET (SGFET).

FIG. 4 is a cross-sectional schematic illustration of a prior art GasFETthat includes a capacitively coupled (CCFET).

FIG. 5 is a cross-sectional illustration of a gas sensor that includes adiffusion gap 16 to limit the diffusion.

Two embodiments shall now be discussed.

A first embodiment uses the property of GasFETs that their operatingtemperatures are drastically lower than those of heated metal oxidesensors, and typically lie between room temperature and about 100° C. Inthis embodiment, a filter 14 is mounted at only a slight physicaldistance from the actual sensor element. FIG. 1 illustrates a typicalstructure. Inasmuch as the operating temperature of the GasFET is about100° C. at most, as mentioned above, the filter 14 can assume theoperating temperature of the GasFET without an unacceptable decrease inits adsorbent activity.

A suitable filter material is carbon fiber fabric, for example that soldby Charcoal Cloth under the trade name Zorflex™. The fabric is made frompure viscose cellulose that is completely carbonized under appropriatereaction conditions. Because of its structure, the material isdistinguished by a large active surface area, with the active surfacebeing produced essentially by the micropores within the fibers. Otherproperties are high mechanical flexibility and stability, low weight,and high chemical resistance. Since the filters are made as a fabricbefore carbonization, the properties can be varied over a wide range,for example by fiber length and thickness, fabric structure, fabricstrength, etc. The adsorption of gas is described as pure physicaladsorption. Because of this, the material can be regenerated by thedesorption of all adsorbates by raising the temperature by 100° C., forexample. Specific impregnations distinctly increase the filteringactivity for certain acidic or basic gases. These gases are then boundby chemisorption. Such fabrics are used successfully, for example, as aprotective lining, for water purification, as oil filters forpurification of compressed air, to protect works of art againstcorrosive gases, and as filters in gas masks.

The GasFET sensors are usually equipped with a heating element tostabilize the temperature. This heating element is now used according toan aspect of the invention to heat the structure, the gas sensor withthe filter 14 mounted directly on it, to a temperature higher than theoperating temperature. Typical temperatures are in the range of 200-300°C. Because the filter 14 and the sensor are in direct contact with oneanother, there is strong heat transfer from the GasFET 12 to the filter14, and the latter reaches the temperature of the gas sensor. There isoutgassing of the activated charcoal filter from the thermal activationat this temperature. The adsorbed gases leave the filter, so that itscomplete adsorption capacity is recovered (i.e., the filter isregenerated). Typical periods for performing the regeneration processare from 1 to 20 days. It typically takes from 0.5 to 3 minutes toaccomplish this regeneration process. No predictive sensor signal mustbe expected from the gas sensor during the regeneration process, bothbecause the GasFET 12 is being operated at too high a temperature, andbecause a certain amount of the interfering gas can also reach theGasFET 12 due to the compelled desorption.

In addition, the measurement signal can be used during the regenerationprocess to monitor the process. Inasmuch as the interfering gasesescaping from the filter usually cause a significant sensor reaction,even at the elevated operating temperatures, there is a sensordeflection attributed to the progress of the regeneration process. Thiscan be used to monitor when the regeneration process has been completed.

A second embodiment is based both on structural details of the FET gassensors and on their property of needing only a very small amount ofanalyte gas for detection, in contrast to the highly gas-consuming metaloxide sensors. To lengthen the service life of the filter elements andto prevent deactivation from saturation, gas access to the filterelement is sharply limited. This greatly delays filter saturation.However, the small amount of available gas is sufficient for gasdetection with the FET sensor. FIG. 2 illustrates a schematicdescription.

Gas diffusion can be limited both by a small orifice as a diffusionlimiter 16 and by a diffusion-limiting membrane that also protects thesystem well against dust. An advantageous embodiment of this principleis illustrated in FIG. 5. Illustrated here is the region of the gasdiffusion gap 16 in a hybrid flip-chip structure for a gas sensor. Adepression is made in the carrier material of the gas-sensitive gate, inwhich the filter material 18 is then deposited. This lengthens thediffusion gap 16. Gas diffusion is then limited by the first region ofthe gas diffusion gap 16. All interfering gas components are almostcompletely removed from the reduced amount of gas by the filter material18, and the purified gas then reaches the detection layer 20.

The two embodiments may be used for cross sensitivities to NOx, NH₃,alcohols, or ketones, which represent important interfering gases forsolid state gas sensors. Very effective adsorption filters areobtainable for these gases. In addition, multiple layers of activatedcharcoal filters can be used. For example, there are activated charcoalfilters impregnated with acidic materials that adsorb basic NH₃ verywell, and filters impregnated with alkaline materials that bind acidicNO₂ very well.

Moisture-adsorbing filter layers may be used, for example like silicagel, to compensate for humidity variations. The objective is not thecomplete removal of moisture, but the use of the filter layer as abuffer that can also emit moisture to smooth out the moisture variationsinterfering with gas detection.

Although the present invention has been illustrated and described withrespect to several preferred embodiments thereof, various changes,omissions and additions to the form and detail thereof, may be madetherein, without departing from the spirit and scope of the invention.

1. A method for increasing the selectivity of an FET-based gas sensorhaving a filter permeable to a target gas and which adsorbs gases on asurface of the filter that cause cross sensitivities to the target gas,where the filter is disposed between a gas mixture to be detected and asensing element of the gas sensor, and where the filter comprisesfabric-like activated charcoal, the method comprising the step ofheating the sensor to a temperature at which the adsorbed gases aredriven off the filter surface to thereby regenerate the filter.
 2. Amethod for increasing the selectivity of an FET-based gas sensor havinga filter permeable to the target gas and which adsorbs gases on asurface of the filter that cause cross sensitivities to the target gas,where the filter is disposed between a gas mixture to be detected and asensing element of the gas sensor, the method comprising the step ofreducing diffusion of the gas mixture to the filter by a predeterminedamount that delays saturation of the filter by a predetermined timeperiod.
 3. The method of claim 1, where an operating temperature of thegas sensor is in the range between room temperature and about 100° C. 4.The method of claim 1, where the filter is positioned at a predeterminedphysical distance from the sensing element.
 5. The method of claim 1,where the fabric-like activated charcoal material comprises viscosecellulose produced by carbonization.
 6. The method of claim 1, where thefilter physically absorbs gases on a surface of the filter that causecross sensitivities to the target gas.
 7. The method of claim 1, wherethe step of heating the sensor comprises the step of heating both thefilters and the sensing element.
 8. The method of claim 1, where thestep of heating the sensor is carried out at a temperature of 100° C.above the operating temperature of the sensor.
 9. The method of claim 1,where the step of heating the sensor is carried out in a temperaturerange of about 150-300° C.
 10. The method of claim 1, where the step ofheating the sensor is controlled automatically.
 11. The method of claim2, where the step of reducing diffusion of the gas mixture is carriedout by one of the steps of providing an aperture in a gas-tight sensorhousing, by providing a gas-permeable membrane in the gas sensor, and byproviding a diffusion gap in the gas sensor.
 12. The method of claim 1,where the filter comprises multiple layers of the fabric-like activatedcharcoal.
 13. The method of claim 1, where the filter is impregnatedwith acidic materials or alkaline materials.
 14. The method of claim 1,where the filter comprises a buffers for moisture.
 15. The method ofclaim 1, where an output signal from the gas sensor is used to control aduration of the step of heating.
 16. The method of claim 1, where anoutput signal from the gas sensor is used during the step of heating tomonitor the regeneration of the filter.
 17. An FET gas sensor,comprising: a filter permeable to a target gas and which adsorbs gaseson a surface of the filter; a sensing element, where the filter isdisposed between a gas mixture to be detected and the sensing element;and a heater that heats the sensor and the filter to a temperature atwhich the absorbed gases are driven off the filter surface to therebyregenerate the filter.
 18. An FET gas sensor, comprising: a filterpermeable to a target gas and which adsorbs gases on a surface of thefilter; a sensing element, where the filter is disposed between a gasmixture to be detected and the sensing element; and an aperture in agas-tight housing or a diffusion gap in the gas sensor for reducing anamount of diffusion of the gas mixture.
 19. The FET gas sensor of claim17, where the filter comprises activated charcoal.
 20. An FET gassensor, comprising: an activated charcoal filter that is permeable to atarget gas and which adsorbs non-target gases on a surface of thefilter; a gas sensing element, where the filter is disposed between agas mixture to be detected and the sensing element; and an aperture in agas-tight housing or a diffusion gap in the gas sensor for reducing anamount of diffusion of the gas mixture that reaches the filter.
 21. Themethod of claim 2, where the step of reducing diffusion of the gasmixture is carried out by one of the steps of providing an aperture in agas-tight sensor housing or by providing a diffusion gap in the gassensor.